1. Field of the Invention
The present invention generally relates to semiconductor devices and more particularly to a quantum optical semiconductor device having a quantum dot structure therein.
2. Description of the Related Art
In a bulk semiconductor crystal where there is no carrier confinement, the state density of the carriers increases continuously with energy in the form of parabolic curve. In a quantum well structure in which there exists one-dimensional carrier confinement, on the other hand, there appear quantum levels as a result of such a one-dimensional carrier confinement, and the state density is changed to have a stepwise form that changes stepwise with energy in correspondence to the quantum levels that characterize the quantum well structure.
Because of the stepwise state density, the carriers experience restriction with regard to the energy distribution in such a system, and thus, the use of a quantum well structure in an optical semiconductor device such as a laser diode leads to an advantageous feature of sharp and narrowly confined optical spectrum, which is superior to the spectrum of a laser diode that uses a bulk semiconductor crystal. In the case of light-emitting devices including laser diodes, the use of a quantum well structure further provides improvement in the efficiency of optical emission. Further, a quantum well structure can be used also as an energy filter in electron devices having a resonant tunneling barrier such as RHET (resonant hot-electron transistor).
In the quantum-well wire structure in which the degree of carrier confinement is increased further, the state density is changed further, because of the existence of the two-dimensional carrier confinement, such that there appears a maximum of state density in each of the steps at the bottom edge thereof. As a result, the sharpness of the energy spectrum of the carriers is increased further.
In the ultimate quantum dot structure in which the degree of carrier confinement is increased further, there appears a discrete state density distribution as a result of the three-dimensional carrier confinement, and associated with thus, the energy spectrum of the carriers becomes totally discrete in correspondence to the discrete quantum levels.
In the system having such a discrete energy spectrum, transition of carriers occurs discontinuously from a quantum level to another quantum level, even in case the system is held in a room temperature environment where there are caused plenty of thermal excitations. Thus, by using such a quantum dot structure, it becomes possible to realize an optical semiconductor device having a very sharp spectrum even in the case the device is operated in the room temperature environment. Further, the use of such a quantum dot structure realizes a very sharp energy spectrum in an electron device having a resonant-tunneling barrier not only at low temperatures but also at the room temperature in the case the quantum dot structure is used for the energy filter.
Further, quantum dot structures draw attention also in the field of fundamental physics in relation to the bottleneck problem of energy relaxation.
Conventionally, a quantum well structure has been formed relatively easily by forming a very thin quantum well layer by an MBE process or MOVPE process such that the quantum well layer is sandwiched between a pair of barrier layers. In the case of forming a quantum-well wire structure, there is proposed a process of growing a semiconductor layer on a so-called inclined semiconductor substrate having a stepped surface such that the semiconductor layer is grown in each of the steps from the step edge with a limited width and limited thickness. Alternatively, a quantum-well wire structure may be formed by forming a one-dimensional quantum well structure by way of electron-beam lithography.
Thus, one may be motivated to form a quantum-dot structure also on an inclined semiconductor substrate by utilizing the surface steps on the substrate, similarly to the case of the quantum-well wire structure. However, such an approach of extrapolating the conventional process encounters various problems such as difficulty of controlling the steps on such a substrate surface, occurrence of mixing of elements at the quantum-dot interface, and the like. When there is caused a mixing of elements, the desired sharp change of composition is not attained at the quantum-dot surface. Further, the use of a patterning process such as lithography for forming the quantum dot inevitably causes substantial damages in the quantum dot.
Meanwhile, there has been proposed a process of forming quantum dots on a substrate in the form of mutually isolated islands by utilizing S-K (Stranski-Krastanow) mode growth, which appears in the initial phase of heteroepitaxial growth caused in a strained heteroepitaxial system such as the InAs/GaAs system.
For example, there is a report (Leonard, D. et al., Appl. Phys. Lett. 63, pp. 3203–3205, 1993) of successful formation of islands of InGaAs on a GaAs substrate with a diameter of 30–40 nm, by growing an InGaAs layer having an In content of about 0.5 on a GaAs substrate, which has a lattice constant substantially different from the lattice constant of the InGaAs layer.
Further, there is a report (Mukai, K., et al., Jpn. J. Appl. Phys. 33, pp. L1710–L1712, 1994) of forming islands of InGaAs on a GaAs substrate with a diameter of 15–20 nm by using an ALE process such that the InGaAs islands are separated from each other with an interval of about 100 nm.
Further, it is reported that similar quantum dots can be formed also by an MOVPE process (Oshinowo, J., et al., Appl. Phys. Lett. 65, (11), pp. 1421–1423 (1994).
Because such formation of quantum dots in a strained heteroepitaxial system is controlled by the strain energy formed at the heterointerface, the formation of the quantum dots is much more simpler than the conventional process of forming the quantum dots. Further, because the process does not use any patterning process such as an electron-beam lithography, there occurs no such a problem that the obtained quantum dots are damaged during the formation process.
Because the foregoing S-K mode growth relies upon the use of lattice-mismatched material system, a quantum dot formed by the S-K mode growth generally accumulates therein a non-uniform strain characterized by an in-plane compressive strain. Further, the quantum dot accumulates a tensile strain or weak compressive strain in the growth direction.
FIG. 1 shows the construction of a quantum semiconductor device disclosed in the Japanese Laid-Open Patent Publication 9-326506 that uses quantum dots formed by the S-K mode growth process.
Referring to FIG. 1, a number of InAs quantum dots 3b are formed on a GaAs substrate 1 having a (100)-oriented surface via a GaAs buffer layer 2 by the S-K mode growth process, wherein the InAs quantum dots 3b are formed in plural layers on the GaAs substrate 1 and are embedded in a GaAs intermediate layer or barrier layer 3a in each of the layers. Further, the quantum dots 3b of the next layer are grown on the barrier layer 3a burying the quantum dots 3b underneath.
In the example of FIG. 1, each of the quantum dots 3b induces a severe strain in the barrier layer 3a covering the quantum dot 3b particularly at the part contacting the apex part of the quantum dot 3b, and as a result, each quantum dot 3b of the next layer tends to grow on the barrier layer 3a in the part immediately above an underlying quantum dot 3b. Thus, there is achieved an alignment of the quantum dots 3b in the direction perpendicular to the surface of the substrate 1 in the case the growth of the barrier layer 3a and the quantum dots 3b is conducted repeatedly.
Thus, by using the quantum semiconductor structure of FIG. 1, it is possible to construct a quantum optical semiconductor device such as a laser diode, optical amplifier, optical switch, wavelength conversion element, and the like, that constitutes an all-optical network or so-called photonic network.
In the case of using such a quantum optical semiconductor device in a photonic network, it should be noted that the quantum optical semiconductor device is required to have a polarization-free characteristics in view of the polarization-free nature of the optical signals transmitted over the optical fibers.
In the case of the quantum dots that uses the S-K mode growth as explained with reference to FIG. 1, on the other hand, the quantum dots have generally a flat shape and accumulate non-isotropic strain therein as noted before, and because of this, construction of optical semiconductor devices has been successful only in the case the optical semiconductor device is the one that amplifies or emits the optical beam of TE mode, as long as the quantum dots formed by the S-K process are used for the active part of the optical semiconductor device. Thus, it has been difficult to construct a polarization-free device, which is required in the actual optical network systems.
In more detail, a non-isotropic strain applied to a quantum dot induces separation of a heavy hole level from a light hole level in the hole level constituting the valence band, and because of this, there appears an energy difference ΔEl−h between the light hole level and the heavy hole level asΔEl−h≈−2b(εzz−εxx)  (1)wherein b is a negative constant called uniaxial deformation potential while εxx and εzz represent respectively the in-plane strain component acting in the direction parallel to the substrate and a strain component acting perpendicular to the substrate surface. In Eq. (1), the positive value of the strain εzz or εxx represents a tensile strain while the negative value represents a compressive strain.
Thus, in the case of the conventional quantum dots that accumulates a compressive strain in the in-plane direction, the energy difference ΔEl−h takes a positive value and thus, the energy difference between the electron level and the heavy hole level becomes smaller than the energy difference between the electron level and the light hole level. As a result, there occurs optical transition between the heavy hole level and the electron level forming the conduction band as represented in FIG. 2.
Meanwhile, it should be noted that such optical transition occurs in the quantum dot only in the case the electric field component of the incoming optical radiation has a direction perpendicular to the wave vector k of the electron waves in the quantum dot. In a flat quantum dot formed by the S-K growth process, it should be noted that quantization of electrons is caused mainly for the electron wave component perpendicular to the substrate surface, and thus, the wave vector k of the electron wave becomes perpendicular to the substrate surface.
Thus, in the quantum dot formed by the S-K mode growth, the interaction between the incoming optical radiation and the electron wave occurs only in the case the incoming optical radiation is a TE-mode optical beam characterized by the electric field parallel to the substrate surface.
Thus, quantum optical semiconductor devices that use quantum dots formed by the S-K mode growth process generally show remarkable polarization dependence, and because of this, it has been difficult to construct a photonic network, which requires polarization-free optical characteristics for the components constituting the network, by using such conventional quantum optical semiconductor devices unless an additional optical system is provided for compensating for the polarization-dependence of the quantum dots. However, such an additional optical system is complex and increases the cost of the optical network.